Water-soluble MnO nanocolloid for a molecular T ₁ MR imaging: A facile one-pot synthesis, in vivo T ₁ MR images, and account for relaxivities

A facile one-pot synthesis of a water-soluble MnO nanocolloid (i.e., d-glucuronic acid-coated MnO nanoparticle) is presented. The MnO nanoparticle in the MnO nanocolloid was coated with a biocompatible and hydrophilic d-glucuronic acid, and its particle diameter was nearly monodisperse and ranged from 2 to 3 nm. The average hydrodynamic diameter of the MnO nanocolloid was estimated to be 5 nm. The MnO nanoparticle was nearly paramagnetic down to T = 3 K. The MnO nanocolloid showed a high longitudinal water proton relaxivity of r₁ = 7.02 s^-1 mM^-1 with the r ₂/r₁ ratio of 6.83 due to five unpaired S-state electrons of Mn(II) ion (S = 5/2) as well as a high surface to volume ratio of the MnO nanoparticle. High contrast in vivo T₁ MR images were obtained for various organs, showing the capability of the MnO nanocolloid as a sensitive T₁ MRI contrast agent. The suggested three key-parameters which control the r₁ and r₂ relaxivities of nanocolloids (i.e., the S value of a metal ion, the spin structure, and the surface to volume ratio of a nanoparticle) successfully accounted for the observed r₁ and r₂ relaxivities of the MnO nanocolloid.